1. Field of the Invention
The invention relates to a sound source detection system that detects a sound source based on sound collected by a sound collecting device, such as microphones.
2. Description of Related Art
A sound source detection system has two or more microphones that collect ambient sound, and is operable to detect a moving direction, etc. of a sound source (e.g., running sound of a vehicle), based on a delay of arrival, i.e., a difference in arrival time between sounds received by the respective microphones, and so forth. In a system described in Japanese Utility Model Application Publication No. 5-92767 (JP 5-92767 U), electric signals generated from two or more microphones placed at a given interval are passed through bandpass filters for removing frequency components of a low frequency band and a high frequency band, respectively, from the electric signals, to be converted into corrected electric signals. Then, the power of a given frequency band in which characteristics of vehicle running sound appears is calculated from the corrected electric signal, and it is determined that an approaching vehicle is present when the power level is larger than a threshold value. On the other hand, an unnecessary noise component is removed from the corrected electric signals, which are thus converted into noise suppression signals, and a cross-correlation between the noise suppression signals of the two or more microphones is calculated, so as to calculate the approaching direction of the approaching vehicle based on a delay of arrival at which the cross-correlation is at the maximum.
When the vehicle moves at a high speed, the maximum detection distance is desired to be sufficiently long since information on a distant approaching vehicle is needed. However, in the method of detecting an approaching vehicle based on a delay of arrival, or a difference in arrival time between sounds received by a pair of microphones, the distance between the pair of microphones is limited by the vehicle width (consequently, the spatial resolution (the performance at the maximum detection distance) is limited), or aliasing occurs in a target frequency band; therefore, as the distance between the approaching vehicle and the self-vehicle increases, the ability to detect the approaching vehicle is reduced, and it becomes difficult to detect the approaching direction, distance, etc. In particular, when the threshold value is used when making a determination on the presence of an approaching vehicle, it is considerably difficult to appropriately set the threshold value, and the detection accuracy largely varies depending on the threshold value. Also, in the method of detecting an approaching vehicle based on the delay of arrival of sounds received by the pair of microphones, the approaching direction cannot be found unless change in the delay of arrival with time is determined; therefore, it cannot be determined, from a single detection, whether the vehicle is approaching the self-vehicle or moving away from the self-vehicle.